Monitoring cells in energy storage system

ABSTRACT

A system for monitoring an energy storage system composed of multiple cells connected in series has a chain of monitors including at least first and second monitors. The first monitor is configured for monitoring at, least a first cell in the energy storage system to produce first monitored data. The second monitor is configured for monitoring at least a second cell in the energy storage system to produce second monitored data. The first monitor is further configured for transferring the first monitored data to the second monitor for delivery to a controller.

This application claims priority of provisional U.S. patent applicationNo. 60/907,423 filed on Apr. 2, 2007, entitled “VOLTAGE STACKABLE DATAPORT” and incorporated herewith by reference.

TECHNICAL FIELD

The subject matter of this disclosure relates to power supply circuits,and, more particularly, to circuitry and methodology for monitoringseries-connected energy storage cells.

BACKGROUND

Management of an energy storage system, such as an electric battery oran ultracapacitor or supercapacitor energy storage system, is essentialto ensuring long life, efficiency, and reliability of the energy storagesystem and an equipment powered by the energy storage system. Propermanagement requires real time knowledge of cell voltage, i.e. voltage ateach energy storage cell should be permanently monitored. Energy storagesystems include multiple energy storage cells connected in series, i.e.“stacked”, so that individual cells near the top of the stack may be atelevated voltages with respect to the system ground.

FIG. 1 illustrates a battery 10 representing an example of an energystorage system. The battery 10 is composed of multiple cells connectedin series. While a cell arranged closer to the ground terminal is at alow potential, another cell connected at the other side of the batterystack may be at a high potential. For example, eighty-one 4.2V cellsshown in FIG. 1 would cause a total voltage of approximately 340V.Therefore, voltage measuring devices capable of measuring voltages ofindividual cells must withstand very high voltages. Using such highvoltage measuring devices for stacked battery management would beextremely expensive and prone to errors.

Another example of an energy storage system is an ultracapacitor orsupercapacitor system. Ultracapacitors or supercapacitors represent oneof the latest innovations in the field of electrical energy storage, andfind their place in many applications involving mass energy storage,power distribution. They are of particular interest in automotiveapplications for hybrid vehicles and as supplementary storage forbattery electric vehicles. In comparison with classical capacitors,these new components allow a much higher energy density, together with ahigher power density. Ultracapacitors or supercapacitors may be producedbased on a double-layer capacitor technology to increase their chargedensity. However, double layer capacitors have a relatively low maximumvoltage. This necessitates a series connection of cells to supportoperation at higher voltages in order to reach an acceptable powerconversion efficiency.

The higher the cell voltage, the shorter the expected life of a doublelayer capacitor. Therefore, cell voltages in the ultaracapacitor orsupercapacitor system should be monitored to prevent voltages atindividual cells from exceeding maximum values. Also, ultracapacitors orsupercapacitors must be monitored during charging to prevent thecharging voltage from exceeding the rated voltage.

A common method of measuring voltages at high common modes involves a4-resistor difference amplifier having a resistor network arranged as acommon-mode voltage divider. For example, this arrangement is used inthe LT®1990 difference amplifier developed by the Linear TechnologyCorporation, the assignee of the present application. However, matchingresistors in the resistor network is a difficult problem. Mismatchedresistors may compromise measurement accuracy as the common mode voltageincreases. Also, the resistor dividers represent a load on the battery.

Another known method of measuring voltage involves capacitive switching.This method is used for measuring a low voltage, for example, in theLTC®1043 dual switched capacitor building block developed by the LinearTechnology Corporation, the assignee of the present application. In thisblock, a pair of switches alternately connects an external capacitor toan input voltage and then connects the charged capacitor across anoutput port. However, at high common mode voltages, this method wouldrequire high voltage MOSFETs, which are not readily available inmonolithic chips.

A further known method includes direct digitization, and level shiftingof digital information. This type of measurement is described in LinearTechnology Design Note DN341 by Mark Thoren entitled “16-bit ADCSimplifies Current Measurements,” although the Design Note relates tomeasurement of current rather than voltage. The Design Note describes a−48V telecom supply current monitor using 16-bit Delta-Sigmaanalog/digital converter (ADC) for direct current (DC) measurements. Themonitor uses optoisolators as level shifting devices for data transfer.However, this method is appropriate for a small number of measurements.Measuring voltage at a large number of cells would require manyoptoisolators (or transistors) and become cumbersome.

Therefore, it would be desirable to create a simple and efficienttechnique for monitoring voltages at multiple energy storage cellsconnected in series.

SUMMARY OF THE DISCLOSURE

The present disclosure offers circuitry and methodology for monitoringindividual cells in an energy storage system composed of multiple cellsconnected in series. The system includes a chain of monitors having atleast first and second monitors. The first monitor monitors at least afirst cell in the energy storage system to produce first monitored data.The second monitor monitors at least a second cell in the energy storagesystem to produce second monitored data. The first monitor is configuredfor transferring the first monitored data to the second monitor.

In particular, the first monitor may be configured for monitoring afirst group of the cells including the first cell, and the secondmonitor may monitor a second group of the cells including the secondcell.

The first monitored data may represent at least one parameter of eachcell in the first group of cells, and the second monitored data mayrepresent at least one parameter of each cell in the second group ofcells. For example, the monitored data may represent voltage at eachcell, and temperature affecting the cell.

The first and second monitors may be adjacent monitors in the chain thatfurther comprises a third monitor adjacent the second monitor andconfigured for monitoring at least a third cell in the energy storagesystem to produce third monitored data. The third cell monitor may befurther configured for receiving the first monitored data from the firstmonitor via the second monitor and for receiving the second monitoreddata from the second monitor.

In accordance with an embodiment of the disclosure, a monitor maytransfer monitored data to a controller via an adjacent monitor. Thecontroller may produce control data transferred to the monitor via theadjacent monitor.

The control data may be produced in response to the monitored data. Forexample, the control data may prevent an individual cell in an energystorage system from being overcharged when the energy storage system isbeing charged. In response to the monitored data relating to aparticular cell, the monitor may create a shunt that forces a chargingcurrent to bypass that cell. The monitor may have a switch controlled bythe control data to produce the shunt.

In accordance with a method of the present disclosure, a first group ofcells may be monitored by a first monitor to produce first monitoreddata, a second group of cells may be monitored by a second monitor toproduce second monitored data. The first monitored data may betransferred to the second monitor for delivery to a controller.

In the opposite direction, control data may be transferred from thecontroller to the first monitor via the second monitor. The control datamay be transferred in response to the first monitored data to control acondition of a cell in the first group of the cell.

Additional advantages and aspects of the disclosure will become readilyapparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present disclosure are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present disclosure. As will be described, thedisclosure is capable of other and different embodiments, and itsseveral details are susceptible of modification in various obviousrespects, all without departing from the spirit of the disclosure.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentdisclosure can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features,wherein:

FIG. 1 illustrates a battery composed of multiple cells connected inseries.

FIG. 2 illustrates an exemplary monitoring system of the presentdisclosure for monitoring an energy storage system composed of multiplecells connected in series.

FIG. 3 shows an exemplary implementation of a monitoring unit in thesystem of the present disclosure.

FIG. 4 illustrates an exemplary arrangement including a controller andmultiple monitoring units connected in a daisy chain.

DETAILED DISCLOSURE OF THE EMBODIMENTS

The present disclosure will be made using specific examples ofmonitoring circuits. It will become apparent, however, that the conceptof the disclosure is applicable to any circuitry for monitoring anyenergy storage system having multiple cells.

FIG. 2 shows an exemplary monitoring system 100 of the presentdisclosure for monitoring an energy storage system 102 composed of ncells C1 to Cn connected in series between terminals of the energystorage system 102. In the illustrated example, cells C1 to Cn may beLithium-Ion (Li-Ion) batteries commonly used in such applications asconsumer electronics and hybrid electric vehicles. However, as oneskilled in the art would realize, cells C1 to Cn may be any energystorage elements. For example, they may be ultracapacitor orsupercapacitor cells. The cell C1 may be connected to a ground terminal,whereas the cell Cn may be connected to a high voltage terminal of theenergy storage system 102, for example, to a +350V terminal.

The monitoring system 100 includes a chain of m battery monitoring unitsBM1 to BMm, each of which may be configured for monitoring apredetermined number of cells C. The number of cells C monitored by eachunit BM may range from 1 to 16 or higher. This number may be determinedby the ability of the monitoring units BM to withstand voltage. Forexample, the withstand capability of each unit BM may be about 60V.Also, the number of monitored cells C may be limited by a pin count of asemiconductor chip on which a particular unit BM is provided. Forexample, FIG. 2 shows that the monitoring unit BM1 monitors 3 cells C1to C3, the unit BM2 monitors 3 cells C4 to C6, etc., finally, themonitoring unit BMm monitors 2 cells Cn−1 and Cn. The monitoring unitsBM may monitor conditions of cells C by determining such parameters, asvoltage at each cell C, or temperature affecting the cells C.

Each monitoring unit BM may be powered using a cell pack monitored bythe respective unit BM. Alternatively, a monitoring unit BM may have apower supply source independent of the monitored cells C.

Adjacent monitoring units BM may monitor adjacent groups of the cells C.For example, the monitoring unit BM1 may monitor cells C1 to C3, and theunit BM2 adjacent BM1 may monitor cells C4 to C6. Alternatively,adjacent units BM may monitor any predetermined groups of cells C.

Each pair of adjacent units BM is connected by a communication link 104that provides a path for data transfers between the units. As shown inFIG. 2, the communication links 104 may be arranged between the unit BM1and unit BM2, between the unit BM2 and the unit BM3, between the unitBM3 and the next monitoring unit, finally, between the unit BMm−1 andthe unit BMm. Each of the communication links 104 may be configured forproviding serial and/or parallel transfer of data between the adjacentunits BM. Although FIG. 2 shows that each communication link 104 iscomposed of 6 lines, one skilled in the art would realize that thecommunication link 104 may include any number of lines required tosupport data transfer in accordance with a selected serial and/orparallel data transfer protocol. For example, as discussed in moredetail below, the communication links 104 may support a data transferprotocol corresponding to an interface, such as a Serial Peripheral

Interface (SPI), established between the monitoring unit BM1 and anexternal controller that may be linked to the unit BM1 to receivemonitored data from the monitoring units BM1 to BMm and/or supply themwith control data. Alternatively, the external controller may be coupledto any other monitoring unit BM.

Each communication link 104 may support bi-directional transfer ofmonitored and controlled data between the adjacent units BM in a serialand/or parallel fashion. For example, monitored data produced by amonitoring unit BMk may be transferred “down” from the unit BMk to amonitoring unit BMk−1 adjacent to the unit BMk in the chain ofmonitoring units, where the unit BMk may be any monitoring unit providedbetween BM1 and BMm for monitoring cells C arranged farther from theground terminal than the cells C monitored by the unit BMk−1, and closerto the ground terminal that the cells C monitored by the unit BMk+1. Ifthe unit BMk−1 is not connected to the controller, it will transfer themonitored data over the next communication link 104 to a unit BMk−2,etc. Control data may be transferred in the opposite direction from theunit BMk−1 to the unit BMk, and then, from the unit BMk to the unitBMk+1. For example, control data may be transferred in a parallelformat, whereas monitored data may be shifted in a serial format.Alternatively, both the monitored and control data may be transferred ina similar fashion—in a parallel or serial format.

The monitored data may include one or more parameters of a cell or agroup of cells being monitored by the respective monitoring unit BM,such as cell voltage and temperature. The control data may includeaddress information identifying a particular cell or a group of cells,and/or control information indicating an operation to be performed inconnection with a particular cell or a group of cells.

For example, the monitoring system 100 may monitor charging of theenergy storage system 102. As individual cells C of the energy storagesystem 102 may become fully charged at different moments of time, themonitoring system 100 may be controlled to stop or slow charging of aparticular cell when it reaches a predetermined voltage to prevent itfrom being damaged. In particular, during a charging procedure, themonitoring units BM may monitor voltage at each cell C and temperatureaffecting the cell C. Over communication links 104, the monitored datadetermined by a unit BMk may be transferred to an adjacent unit BMk−1,then from the unit BMk−1 to the next unit BMk−2, until the transferreddata reach the unit BM1 coupled to a controller. From the unit BM1 themonitored data are transmitted to the controller that may determine thata particular cell Ci reaches a predetermined voltage at a particulartemperature. Based on this information, the controller may producecontrol data identifying the cell Ci and instructing the monitoringsystem 100 to prevent the cell Ci from being further charged. Thecontrol data are transmitted to the unit BM1 connected to thecontroller. From the unit BM1 the control data are transferred overcommunication links 104 to the next unit BM2, then from the unit BM2 tothe next BM unit, until the control data reaches the unit BMk thatmonitors the cell Ci. The control data may cause the unit BMk to takeappropriate actions in order to prevent the cell Ci from being furthercharged. For example, as described in more detail later, a respectiveswitch in the unit BMk may be controlled to create a charge currentshunt around the cell Ci so as to force the charge current to bypass thecell Ci.

FIG. 3 illustrates an exemplary arrangement of a monitoring unit BMk inthe monitoring system 100. The monitoring unit BMk may be arrangedbetween units BMk+1 and BMk−1 in a chain of monitoring units BM tomonitor a 12-cell pack of the energy storage system 102 composed of 12cells Ci, Ci+1, . . . , Ci+11. To comply with the voltage withstandcapability of the unit BMk, the 12-cell pack Ci to Ci+11 may be selectedso as provide a total voltage less than 60V between terminals V+ and V−of the cell pack. The monitoring unit BMk may be implemented on a chippowered by the voltage provided by the cell pack.

The monitoring unit BMk may include a multiplexer 202 having multipleparallel inputs connected across each of the cells Ci to Ci+11 todetermine voltages at the respective cells Ci to Ci+11. The output ofthe multiplexer 202 may present multiple input cell voltages as a singlesequence of cell voltages.

Switches Si to Si+11 may be respectively connected to cells Ci to Ci+11to control their conditions. In particular, the switches Si to Si+11 maybe controlled to shunt the respective cells. For example, duringcharging of the energy storage system 102, any of the switches S may beswitched into an ON state to provide a bypass for the charge currentaround the respective cell C. As a result, the respective cell C may beprevented from being overcharged. For example, field-effect transistorsmay be used as the switches Si to Si+11. Power resistors Ri to Ri+11 maybe connected to the respective switches Si to Si+11 to limit current andcontrol power dissipation. For example, the resistors Ri to Ri+11 mayhave values around 10-100Ω.

Also, the multiplexer 202 may be connected to a die temperature sensor204 that provides the multiplexer with an input signal indicating theinternal temperature of the chip, on which the unit BMk is formed.Further, the unit BMk may utilize a resistor divider R1, R2 connected tothe chip for determining the temperature external with respect to thechip. The resistor divider may include a temperature sensor resistor R1,such as a resistor with a Negative Temperature Coefficient (NTC), and aresistor R2 connected to a reference voltage source 206 that provides areference voltage such as 1.2V. The resistor divider R1, R2 provides themultiplexer 202 with an input signal indicating the externaltemperature.

The internal and external chip temperature information may be presentedat the output of the multiplexer 202, together with a voltage sequencerepresenting voltages at the cells Ci to Ci+11. An analog-to-digital(A/D) converter 208 may convert the analog output signal of themultiplexer 202 into a digital form. The reference voltage source 206may provide a reference voltage required to perform analog to digitalconversion. Hence, the digital output signal of the A/D converter 208represents data monitored by the monitoring unit BMk includinginformation on voltage at each of the cells Ci to Ci+11 and informationon the internal and external chip temperature affecting the cells Ci toCi+11.

The monitored data are supplied to a results register and communicationcircuit 210 that may perform level shifting and provide transferring ofthe monitored data over the communication link 104 to the unit BMk−1that may monitor the next cell pack composed of the cells Ci−1 to Ci−12connected closer to the ground terminal than the cells Ci to Ci+11monitored by the unit BMk. Alternatively, if the circuit 210 is arrangedin the “bottom” monitoring unit BM1 connected to the controller, thecircuit 210 transfers the monitored data to the controller.

Further, the circuit 210 may be connected by another communication link104 to the unit BMk+1 that may monitor the cell pack composed of thecells Ci+12 to Ci+24 connected farther from the ground terminal that thecells Ci to Ci+11. Over this communication link, the circuit 210 of theunit BMk receives data monitored by the units BMk+1 to BMm, andtransfers the monitored data to the unit BMk−1. For example, the circuit210 may contain a hardwired connection to transfer the data receivedfrom the unit BMk+1 to the unit BMk−1.

In addition, the circuit 210 of the unit BMk may be configured forreceiving data from the unit BMk−1 over the communication link 104. Forexample, control data may be transferred from the unit BMk−1 to the unitBMk. If the control data relate to any of the cells Ci to Ci+11monitored by the unit BMk, the circuit 210 may produce a signal toexecute operation instructed by the control data. For example, thecontrol data received from the unit BMk−1 may request preventing one ormore of the cells Ci to Ci+11 from being charged. In this case, thecircuit 210 may produce a control signal supplied to the gate of therespective switch Si to Si+11 in order to place the switch in an ONstate so as to create a charge current shunt around the correspondingcell. If the data from the unit BMk−1 do not relate to the cells Ci toCi+11, the circuit 210 of the unit BMk transfers the received data tothe unit BMk+1.

FIG. 4 schematically illustrates an exemplary connection between theunits BM1 to BMm, each of which may have an arrangement similar to thearrangement of the unit BMk shown in FIG. 3. In particular, the unitsBM1 to BMm may be connected in a daisy chain, in which the unit BM1 iscoupled to the unit BM2, the unit BM2 is coupled to the next BM unit inthe chain, etc. The V+ terminal of one BM unit may be connected to theV− terminal of the adjacent BM unit. The V− terminal of the unit BM1 maybe connected to the ground terminal of the energy storage system 102.The V+ terminal of the unit BMm may be connected to a high voltageterminal of the energy storage system 102, such as a +350V terminal. Thecell packs connected between the V+ and V− terminals may provide powersupply to the respective units BM.

A microprocessor unit (MPU) 300 may be connected to the unit BM1 toreceive data monitored by the units BM1 to BMm and supply thesemonitoring units with control data. The MPU 300 may be powered from apower supply line independent from the energy storage system 102.Alternatively, the MPU 300 may be powered from the same cell pack as theunit BM1. For example, the monitoring system 100 of the presentdisclosure may be arranged in an electric vehicle to monitor anelectrical battery of the vehicle. In this case, the MPU 300 mayinteract with a central controller of the vehicle. The MPU 300 mayprocess voltage and temperature information indicating conditions ofeach cell in the battery.

Based on this information, the MPU 300 may, for example, control themonitoring system 100 to prevent individual cells from being overchargedwhen the energy storage system 102 is being charged. In particular, eachcell can be safely charged until a predetermined cell voltage is reachedat particular temperature conditions. After exceeding this voltage, thecell may be damaged. To prevent damages to the cells, the MPU 300 mayprocess the voltage and temperature information monitored by themonitoring units BM1 to BMm to determine conditions of individual cellsC when the energy storage system 102 is being charged. When datamonitored by a unit BMk indicate that the voltage at a cell Ci reaches apredetermined level at particular temperature conditions, the MPU 300produces control data with address information identifying the cell Ciand the unit BMk that monitors the cell Ci. Via monitoring units BMpreceding the unit BMk in the daisy chain, the control data aretransferred to the unit BMk to cause a gate control signal that switchesin an ON state the switch Si corresponding to the cell Ci. As a result,a charge current shunt is created around the cell Ci to prevent it frombeing charged.

The BM1 may be connected to the MPU 300 via an interface 302. Forexample, a Serial Peripheral Interface (SPI) developed by Motorola maybe used for connecting the MPU 300 to the monitoring units BM. The SPIprovides a synchronous serial data transfer in a full duplex mode. TheSPI uses a master-slave model, where a master device interacts withmultiple slave devices. For example, the MPU 300 may act as a SPI masterdevice, and multiple monitoring units BM1 to BMm may act as SPI slavedevices. The SPI has four signal lines: Master Output Slave Input (MOSI)used for transfer data from the master device to the slave devices,Master Input Slave Output (MISO) used for transfer data from a slavedevice to the master, Serial Clock (CLK) generated by the master deviceto synchronize MOSI and MISO data, and Chip Select (CS) produced by themaster device to address different slave devices. The MOSI, MISO, CLKand CS signal lines may be provided between the MPU 300 and the unitBM1. The communication links 104 between the adjacent monitoring unitsBM may be arranged to support bi-directional SPI data transfer betweenthe MPU 300 and the respective unit BM.

Hence, the present disclosure provides a monitoring system in which noindividual monitoring unit BM has to withstand the entire stackedvoltage of an energy storage system. Instead, each unit BM shouldwithstand only voltage of a cell pack monitored by that unit. Further,communication and control lines are minimized by configuring eachmonitoring unit chip for communication with its local neighbor ratherthan with a chip at a distant location.

The foregoing description illustrates and describes aspects of thepresent invention. Additionally, the disclosure shows and describes onlypreferred embodiments, but as aforementioned, it is to be understoodthat the invention is capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings, and/or the skill orknowledge of the relevant art.

The embodiments described hereinabove are further intended to explainbest modes known of practicing the invention and to enable othersskilled in the art to utilize the invention in such, or other,embodiments and with the various modifications required by theparticular applications or uses of the invention.

Accordingly, the description is not intended to limit the invention tothe form disclosed herein. Also, it is intended that the appended claimsbe construed to include alternative embodiments.

What is claimed is:
 1. A system for monitoring an energy storage systemcomposed of multiple cells connected in series, comprising: a chain ofmonitors including first and second monitors, the first monitor beingconfigured for monitoring cells in a first group of the cells in theenergy storage system to produce first monitored data, and the secondmonitor being configured for monitoring cells in a second group of thecells in the energy storage system to produce second monitored data fordelivery to a controller, the second monitor being further configuredfor receiving the first monitored data from the first monitor fordelivery to the controller, a first group of switches, each coupled to adifferent one of the cells in the first group of cells and controlled bya control signal produced by the first monitor to control charging ofthe cell to which the switch is coupled, and a second group of switches,each coupled to a different one of the cells in the second group ofcells and controlled by a control signal produced by the second monitorto control charging of the cell to which the switch is coupled, thecontroller being responsive to monitored data for producing control dataidentifying at least one of the multiple cells and instructing the chainof monitors to control charging of the identified cell, the control databeing transferred over the chain of monitors, the first monitor beingfurther configured to receive the control data from the second monitorand determine whether the identified cell is one of the cells in thefirst group of cells, the second monitor being further configured toreceive the control data and determine whether the identified cell isone of the cells in the second group of cells, the first monitorproducing the control signal to control the switch in the first group ofcells that is the identified cell, if the identified cell is in thefirst group of cells, and the second monitor producing the controlsignal to control the switch in the second group of cells that is theidentified cell, if the identified cell is in the second group of cells.2. The system of claim 1, wherein each switch is configured to preventthe cell to which it is coupled from being overcharged when the energystorage system is being charged.
 3. The system of claim 2, wherein eachswitch is responsive to the control signal that controls the switch toprovide a bypass for charge current around the cell to which the switchis coupled when the energy storage system is being charged.
 4. Thesystem of claim 3, wherein each switch includes a transistor controlledto provide a charge current shunt around the cell to which it iscoupled.
 5. The system of claim 1, wherein the first monitored datarepresent voltage at each of the cells in the first group of cells, andthe second monitored data represent voltage at each of the cells in thesecond group of cells.
 6. The system of claim 5, wherein the firstmonitored data further represent temperature around each of the cells inthe first group of cells, and the second monitored data furtherrepresent temperature around each of the cells in the second group ofcells.
 7. The system of claim 1, wherein the second monitor comprises aregister and communication circuit for receiving the first monitoreddata from the first monitor, transferring the first and the secondmonitored data to the controller, and for receiving the control datafrom the controller.
 8. The system of claim 7, wherein the register andcommunication circuit are configured for detecting data relating to theone of the cells in the second group of cells in the control datareceived from the controller.
 9. The system of claim 8, wherein thefirst and second monitors are adjacent monitors in the chain.
 10. Thesystem of claim 9, wherein the chain further comprises a third monitoradjacent the second monitor and configured for monitoring at least athird group of the cells in the energy storage system to produce thirdmonitored data, the third monitor being further configured for receivingthe first monitored data from the first monitor via the second monitor,for receiving the second monitored data from the second monitor, and forproviding the control data to the second monitor.
 11. A method ofmonitoring cells in an energy storage system composed of multiple cellsconnected in series, the method comprising the steps of: monitoring afirst group of cells by a first monitor to produce first monitored data,monitoring a second group of cells by a second monitor to produce secondmonitored data, transferring the first monitored data from the firstmonitor to the second monitor for delivery to a controller, transferringthe second monitored data to the controller, in response to monitoreddata, producing control data by the controller to control charging ofthe cells, the control data identifying at least one cell to becontrolled, transferring the control data to the second monitor,determining by the second monitor whether the control data identify atleast one cell in the second group of cells, if the second monitordetermines that the control data identify at least one cell in thesecond group of cells, producing a second control signal to controlcharging said at least one cell, if the second monitor determines thatthe control data do not identify at least one cell in the second groupof cells, transferring the control data from the second monitor to thefirst monitor, determining by the first monitor whether the control dataidentify at least one cell in the first group of cells, and if the firstmonitor determines that the control data identify at least one cell inthe first group of cells, producing a first control signal to controlcharging of said at least one cell in the first group of cells.
 12. Themethod of claim 11, wherein the control data transferred to the firstmonitor are produced in response to the first monitored data.
 13. Themethod of claim 11, wherein the control data transferred to the firstmonitor controls supplying charge current to said at least one cell inthe first group of cells to prevent said at least one cell from beingovercharged during charging of the energy storage system.
 14. The methodof claim 13, wherein the control data transferred to the first monitorcause the charge current to bypass said at least one cell in the firstgroup of cells during charging of the energy storage system.
 15. Themethod of claim 11, wherein the first monitored data indicate voltage atsaid at least one cell in the first group of cells.
 16. The method ofclaim 15, wherein the first monitored data further indicate temperaturein an area of said at least one cell in the first group of cells.